1. Field of the Invention
The present invention relates to an ion gage using a carbon nano-tube, more specifically to a pressure sensor using the field emission of a carbon nano-tube.
2. Background of the Related Art
A pressure sensor (an ion gage), one of the typical sensor for measuring vacuum, is an essential gate used for measuring the environment of chambers in the semiconductor processing. It is of a great importance in the semiconductor processing to maintain a low pressure in a chamber and precisely measure the pressure since it is closely related to the composition of thin film and the characteristic of devices.
The pressure sensor used in the art of the invention does not utilize a classical pressure measuring method where a force exerted on a wall is measured, but measures the density of gas inside a chamber. Thermal electrons generated by means of a carbon nano-tube (hereinafter, referred to as a “CNT”) are accelerated to ionize gas molecules inside the chamber, and the generated ions are detected to measure electric current, thereby enabling to measure the vacuum state.
FIG. 1 is a schematic sectional view, when in use, of a conventional pressure sensor using a carbon nano-tube. FIG. 2 is a partial enlarged view of the portion A in FIG. 1.
As shown in FIGS. 1 and 2, the pressure sensor includes a collector 20, a grid 12, and an electron emission source (CNT) 15 to thereby form a three-pole structure. For this purpose, a metallic layer 11 and the collector 20 are applied with voltages Vs, Vg that is lower than in the grid 12. The grid 12 is applied with a voltage Vc, which is higher than the metallic layer 11 and the collector 20. At this time, if the potential of the collector 20 is established to be lower than that of the grid 12, most thermal electrons (e−) emitted from the CNT 15 are collided with gas molecules 16 to be ionized and then rapidly decelerated and returned to the grid 12. The ionized gas molecules become cations 19, which move towards the collector 20.
More specifically, cold electrons (e−) are emitted from the tip of the CNT 15, which is electrically connected with the metallic layer 11. The emitted electrons (e−) are accelerated towards the grid 12 by the electric field and pass the grid 12.
At this time, the density of gas molecules 16 is very dilute under vacuum environment. These gas molecules are distributed over a relatively long distance between the collector 20 and the grid 12. The electrons passing the grid 12 are collided with the gas molecules 16 to ionize them. During this course of action, cations 19 are generated. The generated cations 19 move to the collector 20 and are collected. That is, the cations 19 are rushed near the collector 20 to form ion current. The electrons emitted from the CNT 15 and the electrons separated from ion molecules are returned to the grid 12 again.
On the other hand, the generated ion current is discharged to the outside through the collector and amplified by an amplifier (not shown) to a measurable size. That is, the gas pressure is in proportion to the number of gas molecules. As the pressure increases, the number of gas molecules increase to thereby increase the probability of ionization, and thus the sensed ion current is increased proportionally.
However, the above conventional pressure sensor for measuring vacuum embraces the following problems. When the electrons emitted from the CNT 15 and the electrons separated from the ionized molecules are returned to the grid 12, they hit the grid 12, which may be damaged and heated up. Therefore, the CNT 15, disposed below the grid 12, is damaged. Consequently, it results in a degraded durability of the pressure sensor.